1. The Field of the Invention
The present invention relates generally to the field of fiber optic communications, and more specifically, to a hybrid cell usable as an interleaver.
2. The Relevant Technology
The Synchronous Optical Network (SONET) standard defines a hierarchy of multiplexing levels and standard protocols that allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals into a common medium. Currently optical communication is accomplished by what is known as “wavelength division multiplexing” (WDM), in which separate subscriber/data sessions can be handled concurrently on a single optic fiber by means of modulation of each of those subscriber data streams on different portions, a.k.a. channels, of the light spectrum.
The spacing between channels is constantly being reduced as the resolution and signal separation capabilities of multiplexers and de-multiplexers are improved. Current International Telecommunications Union (ITU) specifications call for channel separations of approximately 0.4 nm, i.e., 50 GigaHertz (GHz). At this channel, separation as many as 128 channels can be supported in C-band alone. Each channel is modulated on a specific center frequency, within the range of 1525–1575 nm, with the center frequency of each channel provided by a corresponding one of 128 semiconductor lasers. The modulated information from each of the semiconductor lasers is combined (multiplexed) onto a single optic fiber for transmission. As the length of a fiber increases, the signal strength decreases. To offset signal attenuation erbium doped fiber amplifiers (EDFAs) are used at selected locations along the communication path to boost signal strength for all the channels. At the receiving end the processes is reversed, with all the channels on a single fiber separated (demultiplexed), and demodulated optically and/or electrically.
Optical filters play important roles in handling these optical communications for the telecommunications industry. They perform wavelength multiplexing and demultiplexing of the 128 or more optical channels. They can also be used to gain scale EDFAs by flattening their gain profile. In certain configurations, they can also be used as optical interleavers. Interleavers are used to combine two or more streams of lower-channel-count, periodic signals into a single, high-channel-count signal stream with narrow channel spacing. De-interleavers are used to separate a single, incoming narrow-spaced, periodic optical signal stream with high channel count into two or more widely spaced periodic signal streams with low channel count. These interleavers have traditionally been available in two common designs.
The requirements for optical filters/interleavers used for any of these applications are very demanding. The close spacing between the channels in a WDM/interleaver makes it desirable to design a WDM with flat pass bands in order to increase the error tolerance. This is primarily because the center wavelength of a transmitter slips with temperature. Further, the cascading of the WDM stages causes the pass bands to become narrower at each WDM down the chain. Therefore, the larger the pass bands the greater the shift tolerance of the channel.
A common pure crystal interleaver design typically uses a birefringent crystal pair such as a YVO4 and LiNbO3 crystal combination to achieve the interleaving function with thermal compensation. Other crystal combinations, such as TiO2/YVO4, TiO2/LiNbO3, YVO4/PbMnO4, can also achieve thermal compensation and interleaving functions. LiNbO3 is often a preferred choice of crystal since it is widely available and relatively inexpensive.
Unfortunately, the LiNbO3 crystal has an inherent long-term stability problem probably because of its photorefractive and photoelastic properties. Its index of refraction will drift over time under environmental stress. The pure crystal design also suffers from 2nd order effects, resulting in imperfect thermal compensation over a given temperature range. This problem arises from the fact that the thermal expansion coefficient and thermal optical coefficient (dn/dT) of most birefringent crystals is temperature dependent. Crystal pairs that are thermally compensated at room temperature or at any arbitrary temperature will not be thermally compensated at any different temperature. This leads to an undesirable amount of central wavelength (CWL) drift over the device operating temperature range, which is typically −5 to 70 deg C.
An alternate design for an interleaver uses polarizing beam splitters (PBS) made from a BaK1 and LaSF3 combination. The PBS controls the passage of light through the interleaver based upon the state of the polarization of the light. The PBS can also polarize light to a given state depending on the optical axis of the PBS.
The PBS design only uses optical glass. It will direct light to its respective path according to its polarization direction. Many of the PBS interleaver designs have much better thermal compensation properties since the thermal expansion coefficients and thermal optical coefficients of many types of optical glass used in these interleavers have much less temperature dependence. Most PBS types of interleavers do not suffer from the problem of CWL drift over operating temperature range as crystal interleavers do.
However, in some cases, for example, a 100 GHz interleaver, the PBS can be too large or small depending on the type of glass selected, leading to undesirable device size or aperture clipping. Moreover, many of the types of glass selected for proper device size are not well established, and therefore, expensive. The properties of such glass have not been scientifically established. Furthermore, the properties can be inconsistent from batch to batch, resulting in manufacturing difficulties. Thus, it is desirable to use an optical glass, which is well established in the industry, has a low cost, and is consistent in optical quality. Further, it is also desirable to use a well-known optical material to achieve thermal compensation while having easy to handle dimensions (a few mm in cross sections, small enough, yet easy to handle).